The present invention relates to a movement detection circuit used in a color television receiver, and more particularly to a movement detection circuit suitable for detecting movement information of image in a television receiver which receives an NTSC composite color television signal.
In the NTSC television system, a chrominance signal is transmitted by a sub-carrier modulated by the chrominance signal. The sub-carrier is supermmposed on an area of 2.1-4.2 MHz in a 4.2 MHz video signal band. A frequency f.sub.sc of the sub-carrier and a horizontal scan frequency f.sub.H have a re1ationship of ##EQU1## and the horizontal scan frequency f.sub.H and a vertical scan frequency f.sub.V have a relationship of ##EQU2## A luminance signal and the chrominance signal are frequency-interleaved with each other.
A phase of the sub-carrier is reversed at an interval of one frame period. Thus, when two composite signals spaced by one frame period are added, the luminance signal is produced, and when they are subtracted, the chrominance signal is separated. Thus, for a still image, a cross-component such as cross-color or dot disturbance is eliminated and a substantially perfect luminance signal and chrominance signal are produced and the quality of image of the television receiver is improved.
However, when the two signals spaced by one frame period are processed for dynamic images, double images appear on a picture screen of the television receiver or an elimination effect for the cross-component disappears and the dot disturbance appears. As a result, the image quality is deteriorated.
JP-A No. 55-123280 discloses an apparatus for detecting movement of an image by detecting a difference between two scan line signals spaced by one frame period. This apparatus detects that the movement of the detected image is small, that is, the image is a still image when the differential signal is small and processes the two signals spaced by one frame period to separate the luminance signal and the chrominance signal. When the differential signal is large, it detects that the movement of the image is large, that is, the image is a dynamic image, and scan line signals are processed in the field to separate the luminance signal and the chrominance signal. This apparatus is known as a movement adapted signal processor.
As described above, the phase of the chrominance sub-carrier is reversed at an interval of one frame period. Accordingly, if a difference between two composite color televisionssignals spaced by one frame period is simply determined, a differential signal based on the movement of the image and the differential signal based on the phase difference of the sub-carrier are detected in mixture. As a result, the still image may be detected as the dynamic image by the presence of the chrominance signal.
In the prior art, the differential signal of the chrominance signal band centered at the frequency f.sub.sc is eliminated from the differential signal of the two scan lines spaced by one frame period by the lowpass filter, and only the differential signal of the luminance signal is detected and the movement of the image is detected.
On the other hand, a technique has been known where an interlace scan signal used in the TTSC system is converted to a sequential scan signal to display the image. This apparatus uses a field memory and an interpolating scan line signal is generated by using a one-field prior scan line signal to produce the sequential scan signal. This apparatus can eliminate a line flicker generated at edges of a lateral line included in the image. The interpolation of the signal between the fields has a large effect on the still image but it creates an interdigital double-image to the dynamic image.
JP-A No. 58-205377 discloses an adaptive processor which detects the movement of the image based on the differential signal of two signals spaced by one frame period, and when the movement of the image is small, a signal to be interpolated is generated based on the signals of different fields, and when the movement of the image is large, the interpolating scan line signal is generated by using the scan line signal in the same field.
As shown in FIG. 2, when an image J of an object moves from a top to a bottom of aniimage screen P, the movement of the object image is represented as shown in FIG. 3 by using an interlace scan line, where an ordinate is along the vertical direction of the screen and an abscissa represents a time axis.
In FIG. 3, circles represent scan lines l (in sectional view). When an interpolating scan line of an M-th field shown by a triangle is to be generated, it is necessary to generate the interpolating scan line based on a signal in the same field because the image is a dynamic image. However, as sen from FIG. 3, a differential signal between a signal on a scan line l.sub.M-1 in the (M-1)th field and a signal on a scan line l.sub.M+ in the (M+1)th field is zero because both scan line signals are equal. Accordingly, the movement (change) of the triangle scan line signal is not detected from the differential signal of the one-frame spaced signals. As a result, the prior art circuit effects to the dynamic image the interfield interpolation which is to be done to the still image, that is, generates the interpolation signal based on the signals in the (M-1)th field and the (M+1)th field. As a result, the image quality is deteriorated.
As shown in FIG. 4(a), when the object image J moves vertically, the movement information detected from the interframe differential signal indicates movement in areas (L.sub.0 -L.sub.1) and (L.sub.2 -L.sub.3), and non-movement in an area (L.sub.1 -L.sub.2) as shown in FIG. 4(b). Since the detection is in error in the non-movement information area (L.sub.1 -L.sub.2), a correction is usually made, and corrected movement information as shown in FIG. 4(c) is produce.
However, in the prior art, when two frames W.sub.1 and W.sub.2 move slowly as shown in FIG. 5(a), the detected movement information is the same as that shown in FIG. 4(b), and the corrected movement information is the same as that shown in FIG. 4(c). As a result, an object image S between the frames W.sub.1 and W.sub.2 a is processed as the dynamic image although it is a still image.
The technique disclosed in JP-A No. 55-123280 cannot correctly detect the movement information of the image in which only the chrominance signal changes. As a result, the still image processing is effected for the image whose color changes, and the image quality is deteriorated.
The technique disclosed in JP-A No. 58-205377 controls the processing of the interfield signal by detecting the two signals spaced by one frame period but it cannot correctly detect the fast movement of the object. A movement detection circuit for detecting the movement information of the chrominance signal is disclosed in U.S. patent application Ser. No. 932376.